Equilibration of lower alkyl substituted polyalkyl aromatic hydrocarbons with reduced disproportionation

ABSTRACT

Disproportionation losses in the isomerization of C 8  aromatic hydrocarbons are reduced by the use of silica-alumina catalyst composites with high surface areas and having .[.pore.]. .Iadd.most of its pore volume from pores of .Iaddend. radii .[.in the main.]. above about 45° A.

FIELD OF INVENTION

This invention relates to a method for the improvement of the catalyzedisomerization of lower alkyl polyalkyl-substituted aromatichydrocarbons, particularly to the equilibration of non-equilibriumxylene feeds effected by silica-alumina composites. More particularly,it relates to novel catalyst parameters which result in a substantialreduction in the loss to disproportionation by-products normallyexperienced in the subject isomerization process.

BACKGROUND OF INVENTION

It is known to isomerize lower alkyl-substituted aromatic hydrocarbonsat elevated temperatures by passing the vaporized feed into contact witha solid silica-alumina catalyst composite for the purpose of alteringthe isomer distribution of the hydrocarbon feed. The usual objective isto convert the feed into a mixture of isomeric compounds which morenearly approximates the known isomer equilibration composition for thespecific feed composite or mixture. In practice, the foregoingequilibration process is coupled with a subsequent separation processutilizing physical or chemical means for the preferential and selectiveremoval of a desired isomer from the equilibrate. By recycle of theresidue from the separation stage and repetition of the overalloperation one is able to convert a single isomeride or isomeric mixtureto a desired specie of the equilibration set.

An undesirable aspect in conventional practice in the foregoing stagedconversion and recovery cycle is that a substantial fraction of theprocess feed is lost because of a concurrent disproportionationreaction. For example, in a process for the production of p-xylene inwhich a p-xylene-reduced xylene mixture is isomerized in an extinctionrecycle process, the formation of disproportionation products such asbenzene, toluene, and trimethyl benzenes represents an appreciable lossof xylene product or a large process burden where these components ofthe product stream are recycled to the process together with fresh feed.In a typical petroleum refinery operation it is usually preferred not torecycle such by-products. Consequently, as much as 30-40% of the xylenefeed is converted through the disproportionation reaction to lessdesirable product.

.Iadd.SUMMARY OF THE INVENTION .Iaddend.

It has now been found that in the isomerization of lower alkylpolyalkyl-substituted aromatic hydrocarbons relative rate differencesbetween the concurrent isomerization and disproportionation reactions ofthese feeds can be effectively utilized by the employment of asilica-alumina catalyst composite having a surface area in the range250-400 square meters per gram--preferably 270-350 square meters pergram--and wherein at least 40% of the .[.pores.]. .Iadd.pore volume.Iaddend.of the catalyst .[.have.]. .Iadd.is from pores having.Iaddend.a radius greater than 45 A., preferably greater than 50 A., andless than about 100 A. Surprisingly, the use of the isomerizationcatalysts herein results in a large reduction in the disproportionationloss experienced with conventional silica-alumina isomerizationcatalysts. As much as 40 percent and more of the former loss todisproportionation products is eliminated by employing the high surfacearea-high pore radius silica-alumina catalysts of the present invention.

By lower alkyl aromatic hydrocarbon isomerization conditions is meant atemperature in the range 700° F. to 1000° F., preferably from about 750°F. to 900° F., and at a liquid hourly space velocity in the range 0.1 to10, and at a pressure below about 200 p.s.i.g., preferably below about50 p.s.i.g.

Aromatic hydrocarbons having an aromatic carbocyclic carbon atom content(ring carbon atoms) in the range below about 13 and which have from 2 toabout 4, preferably 2 to 3, lower alkyl groups are satisfactory feedsfor the process herein.

By a lower alkyl group is meant an alkyl group having a carbon atomcontent of less than 4.

In general, isomerization catalysts comprising silica-alumina which havea surface area in the range 250-400 square meters per gram and for whichat least 40 percent of the .[.pores have.]. .Iadd.pore volume is frompores having .Iaddend.a radius greater than 45 A., are satisfactory foruse in the instant process.

In a preferred embodiment of the invention the mother liquor remainingafter removal by crystallization of p-xylene from an isomeric xylenemixture, i.e., a p-xylene reduced xylene feed, is used as a feed sourcefor a catalyzed xylene isomerization stage in which the catalyst is asilica-alumina composite containing about 80 percent of silica, about 20percent of alumina, the foregoing percentages being by weight, a surfacearea of 270-350 square meters per gram, with at least about 70 percentof the .[.pores having.]. .Iadd.pore volume is from .Iaddend. a radiusgreater than 45 A., and with the particle sizing being in the range0.06-0.1 inch. The isomerization is carried out in a fixed bed reactorsystem under the following conditions:

    ______________________________________                                        Temperature, ° F.                                                                             750-900                                                Pressure, atmosphere   1.1                                                    Co-feed water-ammonia:                                                        Water, p.p.m. of xylene                                                                              5 × 10.sup.4                                     Ammonia, p.p.m. of xylene                                                                            100                                                    ______________________________________                                    

The effluent from the above isomerization stage is cooled as requiredand passed to a stripper column where water and the light aromaticsproduced in the isomerization are taken as an overhead fraction and thebottoms portion is passed to a second distillation column from which thexylene fraction is recovered as an overhead stream while the heavyaromatic, trimethyl benzenes and the like, produced in the isomerizationare withdrawn from the process or a fraction thereof is recycled to theisomerization stage.

The silica-alumina catalysts useful herein are rigid three dimensionalnetworks characterized by uniform pores having relatively large porediameters. It appears that the combination of physical characteristics,the particular surface area range and uniformly high pore radius, of thecatalyst permits a diffusivity relationship favoring the desiredisomerization reaction relative to the unwanted disproportionationreaction within the liquid hourly space velocity range specified above.

Preferred catalyst composites contain silica and alumina in the weightratio 70-90 and 10-30 respectively, preferably about 80 to 20.

In addition, useful composites may contain minor amounts, about 5 to 15parts, of such metal oxides as magnesia, zirconia, thoria, beryllia, andthe rare earth metal oxides.

Silica-alumina catalyst composites having the necessary characteristicsfor use herein are conveniently prepared by the simultaneousco-precipitation or co-gellation (co-gel method) of a mixture ofcompounds of silicon and aluminum. The surface areas of the dried orpartially dried composites are in general steamed-down to the usefulrange at temperatures in the range 500-1300° F., surface areadeterminations being made by the well known B.E.T. Method using nitrogen(cf. G. M. Schwab, "Handbuck der Katalyse," volume 4, page 195).

Representative and suitable co-gel-type preparative methods are to befound in the procedures as disclosed in U.S. 3,280,040, U.S. 3,399,132and U.S. 3,401,125, except that the material other than the precursorsfor the silica, alumina, or other metal oxide components contemplatedherein are omitted. General background information is to be found in thepaper, "Control of Physical Structure of Silica-Alumina Catalyst," K. D.Ashley and W. B. Innes, Industrial & Engr. Chem., volume 44, 1952. Onthe other hand, the pH ranges for the co-gellation, working techniquesincluding ion exchange and base exchange means as disclosed are ingeneral desirably employed as disclosed in the above patent references.

Representative aromatic hydrocarbon feeds desirably employed in theprocess of the invention include p-xylene reduced (i.e., containing lessthan the equilibrium amount) C₈ aromatic hydrocarbon mixtures, m-xylene,p-xylene, o-xylene, ethylbenzene, o-cymene, p-cymene, m-cymene,non-equilibrium enzyme mixtures, m-diisopropylbenzene,p-diisopropylbenzene, p-diethylbenzene, m-diethylbenzene,3-isopropylethylbenzene, m-(n-propyl)-toluene, misitylene,1,3-dimethylnaphthalene, 1,4-diethylnaphthalene,1,5-diisopropylnaphthalene, 1,3,5-trimethylnaphthalene, and the likepolyalkyl aromatic hydrocarbons. The polymethylbenzenes and mixturesthereof are preferred feeds.

The following examples further illustrate the invention.

EXAMPLES 1-3

A series of silica-alumina catalysts (80-20 weight ratio) were preparedby the co-gellation method and extruded (one-tenth inch diameter) andtempered. Three catalysts having the characteristics:

    ______________________________________                                                              Percent                                                                       .[.pores >45°.].                                             Surface   .Iadd.pore volume.Iaddend.                                          area,     .Iadd.from 45A..Iaddend.                                            M.sup.2 /g.                                                                             radius                                                  Catalyst:                                                                     A             293          0                                                  B             327         20                                                  C             274         68                                                  ______________________________________                                    

were tested for disproportionation activity using a xylene feed with thefollowing results:

    ______________________________________                                        Catalyst:     Relative disproportionation                                     ______________________________________                                        A             1.0                                                             B             .71                                                             C             .60                                                             ______________________________________                                    

In the foregoing comparison, the process feed had the hydrocarboncomposition:

    ______________________________________                                                              Wt. percent                                             m-Xylene                53.3                                                  Ethylbenzene            25.2                                                  p-Xylene                8.8                                                   o-Xylene                6.6                                                   Toluene                 4.7                                                   Other (paraffins and naphthenes)                                                                      1.4                                                   ______________________________________                                    

and contained in addition 500 p.p.m. (based upon hydrocarbon) of ammoniaand 2.5 × 10⁴ p.p.m. of water. The reaction conditions were:

    ______________________________________                                        Temperature, ° F.                                                                             750                                                    Pressure, ATM           1                                                     LHSV, v./v./hr.:                                                              3 hr. at 1.76                                                                 18.5 hrs. at 0.88                                                             ______________________________________                                    

These data demonstrate that for a minor disproportionation activity.[.the pore radius.]. .Iadd.at least 40% of the pore volume .Iaddend.ofa silica-alumina catalyst composite should in the main be .Iadd.frompores having a pore radius .Iaddend.above about 45 A., that is few ornone (less than 10 percent) of the catalyst .[.pores should have.]..Iadd.pore volume should be from pores having .Iaddend.a radius lessthan about 45 A.

EXAMPLE 4

A silica-alumina catalyst composite was prepared which corresponded tocatalyst C in the above examples except that sufficient of magnesiumchloride was included in the co-gellation preparative mixture to providefor 5 parts of magnesia in the resulting catalyst. This catalyst had asurface area of 336 square meters per grams, with about 69 percent ofthe .[.pores.]. .Iadd.pore volume .Iaddend.thereof .Iadd.from pores.Iaddend.having a pore radius greater than 45 A. The isomerization anddisproportionation activities were substantially the same as forcatalyst C. However, the magnesia additive had reduced the cokingactivity (rate of coke lay-down) by about one-half of that for catalystC.

The foregoing demonstrates that the addition of a minor amount ofmagnesia to a silica-alumina isomerization catalyst effectively reducescoke formation without impairing the isomerization activity orincreasing the disproportionation activity of the catalyst.

What is claimed is:
 1. The process for the catalytic isomerization of analkyl substituted polyalkyl aromatic hydrocarbon feed which comprisescontacting the feed with a silica-alumina isomerization catalyst at atemperature in the range from about 700° F. to 1000° F., at a liquidhourly space velocity in the range from about 0.1 to 10, and at apressure below about 200 p.s.i.g., wherein said catalyst consistsessentially of a silica and alumina composite having a surface area inthe range from about 250 to 400 square meters per gram, wherein at least40 percent of the catalyst .Iadd.pore volume is from .Iaddend.pores.[.have.]. .Iadd.having .Iaddend.a radius greater than about 45 A., saidfeed having an aromatic carbocyclic carbon atom content below about 13,and having from 2 to about 4 of the same or different lower alkylsubstituent groups; and said composite having a silica to alumina weightratio in the range from about 70-90 to about 10-30, respectively.
 2. Theprocess as in claim 1 further characterized in that the feed is apolymethylbenzene, in that the surface area is in the range from about270-350 square meters per gram, and in that at least about 70 percent ofthe .Iadd.pore volume is from .Iaddend.pores .[.have.]. .Iadd.having.Iaddend.a radius greater than 45 A.
 3. The process as in claim 2further characterized in that less than about 10 percent of the.Iadd.pore volume is from .Iaddend.pores .[.have.]. .Iadd.having.Iaddend.a radius less than 45 A.
 4. The process as in claim 2 furthercharacterized in that the composite contains, in addition to the silicaand alumina, an amount in the range from about 5 to 15 parts by weightof an oxide selected from the group consisting of magnesia, zirconia,thoria and beryllia.
 5. The process as in claim 4 further characterizedin that the oxide is magnesia and in that the silica to alumina weightratio is about 80 to 20, respectively.
 6. The process as in claim 1further characterized in that the composite contains, in addition to thesilica and alumina, an amount in the range from about 5 to 15 parts byweight of an oxide selected from the group consisting of magnesia,zirconia, thoria, and beryllia.
 7. The process as in claim 5 furthercharacterized in that the feed is a p-xylene reduced C₈ aromatichydrocarbon mixture.
 8. The process as in claim 1 further characterizedin that the pore radius is greater than 50 A.
 9. The process as in claim1 further characterized in that the temperature is in the range fromabout 750° F. to 900° F.